A wireless power transfer (WPT) system wirelessly transfers power through the air to maximize convenience of supply of power to mobile devices and digital appliances.
The WPT system has advantages such as energy conservation through real-time power use control, overcoming of power supply space restriction and reduction of the amount of waste batteries due to battery recharge.
The WPT system may be implemented using a magnetic induction method or a magnetic resonance method.
The magnetic induction method is non-contact energy transfer technology of placing two coils close to each other, enabling current to flow in one coil to generate a flux and generating electromotive force in the other coil due to the flux and may use a frequency of several hundred kHz.
The magnetic resonance method is magnetic resonance technology of utilizing an electric field or a magnetic field without using an electromagnetic wave or current, may transfer power over a distance equal to or greater than several meters, and may use a band of several tens of MHz.
Such a WPT system requires a direct current (DC) voltage.
Examples of a voltage supply for supplying an appropriate voltage to the WPT system include a DC-DC converter, an AC-DC power supply, a DC-AC inverter, etc.
The power supply is designed to satisfy high efficiency and low cost and a structure for changing the level of a necessary voltage is being developed using various methods.
In the WPT system, in power transfer efficiency, impedance matching, decision of a target wireless power transfer system-device, etc. between a wireless power transfer system-charger for transferring power and a wireless power transfer system-device for receiving power, it is important to change the level of the voltage of the power supply to generate a voltage necessary for the WPT system.
In the related art, since circuits for sensing the voltages or currents of coils included in a wireless power transfer system-charger and a wireless power transfer system-device and a circuit for controlling the level of a voltage according to communication between the wireless power transfer system-charger and the wireless power transfer system-device are separately provided, the volume of the system increases and complexity of the circuit increases.
In particular, in a DC-DC converter for outputting a constant voltage, the values of elements included in the converter should be changed in order to change the voltage. Therefore, complexity of the circuit is increased.